第一方法：

def read_in_chunks(filePath, chunk_size=1024*1024):
    """
    Lazy function (generator) to read a file piece by piece.
    Default chunk size: 1M
    You can set your own chunk size 
    """
    file_object = open(filePath)
    while True:
        chunk_data = file_object.read(chunk_size)
        if not chunk_data:
            break
        yield chunk_data
if __name__ == "__main__":
    filePath = './path/filename'
    for chunk in read_in_chunks(filePath):
        process(chunk) # <do something with chunk>

第二種方法：

#If the file is line based
with open(...) as f:
    for line in f:
        process(line) # <do something with line>

第三種方法：

import fileinput
for line in fileinput.input(['sum.log']):
    print line